This invention relates to a wear-resistant alloy which can be suitably used not only as an erosion shield provided for the terminal blade of the low pressure section of a turbine used with an atomic power plant but also as the sliding parts of control rods.
As is well known, a boiling water type atomic power plant is a system for generating power by revolving a turbine, using steam produced in a nuclear reactor. With the boiling water type nuclear reactor, water is heated into steam, which in turn is conducted through a main steam pipe to a turbine for its revolution. Steam gradually increases in humidity while being circulated for revolution of a turbine. Wet steam is conducted to a condenser after leaving a turbine to be converted into water. The water is returned to the reactor after being preheated by a feed water heater.
In the atomic power plant, some parts are subject to little wear such as a pipe while used as a main steam pipe, other pines provided for a condenser and feed water heater, the blades of the high pressure section of a turbine and the casing thereof are generally prepared from, for example, 18-8 stainless steel. On the other hand, parts subject to severe wear comprising erosion by high speed steam streams or violent cavitation erosions, such as, for example, the erosion shield provided for the terminal blade of the low pressure section of a turbine, the face section of valves, the sliding section of control rods and parts of a jet pump, should be built of wear-resistant material. These parts undergoing heavy erosions are generally formed of a cobalt-chromium-tungsten alloy sold under the trademark Stellite containing about 50% by weight of cobalt. However, the above-mentioned steel material and a cobalt-chromium-tungsten alloy sold under the trademark Stellite are gradually corroded or eroded during long use, giving rise to the growth of corrosion or erosion refuse such as ions or fine particles of metals. This corrosion or erosion refuse is accumulated in a reactor by circulation of steam or water.
When bombarded by neutrons emitted from fuel rods the corrosion or erosion refuse is presumably converted into radioactive corrosion or erosion product. Radioactive corrosion or erosion product arising from steel material has a very short half life, whereas radioactive corrosion or erosion product whose nucleus is formed of cobalt 60 derived from cobalt 59 contained in a cobalt-chromium-tungsten alloy sold under the trademark; Stellite has a relatively long half life. Radiation sent forth from said radioactive corrosion or erosion product increases in amount as the run of an atomic power plant is prolonged. Therefore, it sometimes happens that when a periodic maintenance or repair of an atomic power plant is undertaken, the atomic power plant has to be stopped for a considerably long period in order to wait for the sufficient attenuation of radiation issuing from radioactive corrosion or erosion product deposited in the atomic power plant.
Hitherto, therefore, demand has been made to develop a wear-resistant material free from an element such as cobalt which gives rise to the growth of radioactive corrosion or erosion product having a long half life, in order to shorten the rest period of an atomic power plant as much as possible for its efficient operation.